Source:

Response:

Sandstones, shales and other formations often do not have uniform ages.
For example, the extensive St. Peter Sandstone of central North America
was deposited at different times in different locations.

Shales form mostly from mud on the ocean floor, which does cover large
areas. In the late Ordovician, much of North America was covered by a
shallow sea. Much shale formed there over millions of years, to be
exposed when the sea level lowered.

Sands occur mostly along shorelines. When a shoreline recedes
gradually, sands can be left covering a large area.

A catastrophic flood would not be expected to produce such large
amounts of shale and sandstone. The particle sizes in these sediments
is uniform; the gravel, sand, and mud have been sorted apart into
different areas. The high energies in the flood would mix everything
together. At best, a flood could redeposit sands or muds that already
existed, and it would take millions of years for such quantities to
form.

Furthermore, shales are sometimes found atop sandstones. A single
flood could not deposit both. Even more impossible for a single flood,
we also see multiple layers of sand or shale interleaved with other
materials, such as volcanic ash [Nanayama et al. 2003].